Multiple reflector antenna with offset feed



Sept. 22, 1970 RAVENSCRQFT 3,530,476

MULTIPLE REFLECTOR ANTENNA WITH OFFSET FEED Filed June 29. 1967 2 Sheets-Sheet l HIGH POWER TRANSM/ TTERS MISC EQUIPMENT AZ/MUTH DRIVE 5 2 1m? A. Pfl/ Y OM f INVENTOR BY 74%? 29am ATTORNEY P 1970 l. A. RAVENSCROFT 3,530,476

MULTIPLE REFLECTOR ANTENNA WITH OFFSET FEED- Filed June 29. 1967 2 Sheets-Sheet 2 "-$UB-REFLECTOR AXIS RAY PATHS F/eJ.

INVENTOR BY fl f fl ATTORNEY United States Patent U.S. Cl. 343761 9 Claims ABSTRACT OF THE DISCLOSURE An aerial having a symmetrical paraboloidal main reflector and a sub-reflector having a reflecting surface defined by part of a hyperboloid one focus of which coincides with the main reflector. The aerial also includes a primary feed horn having a phase centre at the other focus of the hyperboloid 'which is depressed relative to the axis of the main reflector. In a plane containing the axis symmetry of the hyperboloid and of the paraboloid the sub-reflector has an offset, hyperbolic shaped section. In an orthogonal plane, the sub-reflector section is a symmetrical hyperbole.

This invention relates to aerial systems suitable for use, for example, in satellite communications systems.

Satellite communications systems are currently in practical operation and several types of aerial systems have been used for transmitting information to, and receiving information from, satellites in various orbits.

In the United Kingdom, a symmetrical paraboloidal reflector, with its focus in the aperture plane, has been constructed and used at Goonhilly, relevant details appearing in the Post Oflice Electrical Engineering Journal, July 1962, page 105. This aerial has been used in conjunction with Projects Telstar and Relay with satisfactory results. In this aerial the elevation axis intersects the principal axis of the paraboloid. Electrical equipment associated with the aerial, including a liquid helium cooled low-noise maser amplifier, is housed behind the paraboloid close to the elevation axis. Of necessity, there is a relatively long length of waveguide between this equipment and the aerial feed assembly (located at the focus of the paraboloid), which results in a higher electrical loss and a higher noise temperature than is desirable being associated with this form of aerial. In addition, access to the helium cooled amplifier, housed in a cabin which has to be tilted with the aerial, is diflicult under working conditions.

Aerial systems employing Cassegrain principles have been considered as suitable for use in communications systems. However, using a normal symmetrical Cassegrain system, although a long waveguide run can be avoided, access is required near the vertex of the main reflector (which can be inconvenient), and it also suffers from the disadvantage that at low angles of elevation, spillover past the sub-reflector'which is necessary for efiicient illumination of the main reflector-- can be troublesome to other stations and, more importantly, results in receiving equipment coupled to the aerial being vulnerable to high temperature noise.

The present invention is concerned with an aerial systern, suitable for use with communications satellites, that offers improvements over the above discussed aerial systerns.

The present invention is an aerial system including a concave main reflector symmetrical about the axis of revolution of its generating curve and a sub-reflector shaped and sited to direct energy received by reflection from the main reflector to a primary feed device offset from the said axis of revolution. That is to say, the mean direction of such energy reflected by the sub-reflector is inclined to the said axis of revolution. The sub-reflector will also function to direct towards the main reflector energy transmitted from the primary feed device.

In a preferred form, the present invention is an aerial system including a paraboloidal main reflector symmetrical about the axis of the generating parabola and a sub-reflector located adjacent the focus of the main reflector, the sub-reflector forming part of a hyperboloid one focus of which is coincident with that of the paraboloid the other focus being depressed relative to the paraboloid axis and nearer the main reflector surface, and a primary feed device operable to illuminate the sub-reflector, having a phase centre at the said other focus. The sub-reflector will also direct energy received by reflection from the main reflector to the primary feed device.

Thus, in aerial systems embodying the invention, in the plane containing the axis of the sub-reflector hyperboloid and the axis of the paraboloid, the sub-reflector has an offset hyperbolic shaped section, whilst in the orthogonal plane the section is a symmetrical hyperbola.

Alternative primary feed arrangements can be accommodated by location of alternative primary feeds having phase centres on the circle of rotation of the said other focus of the sub-reflector hyperboloid about the axis of the main reflector. In order to switch from one primary feed to another, the sub-reflector is rotated by an appropriate amount about its principal axis. This arrangement permits simple switching techniques to be used for effecting changeover from one feed to another.

As is usual in such an aerial system, the main reflector can be rotated together with the sub-reflector both in elevation and in azimuth, and the elevation axis passes through the axis of the hyperboloid of which the subreflector forms part. Use of a relatively short waveguide run to connect the primary feed to a low-noise (e.g. maser) amplifier, preferably located on the elevation axis, then permits the reflector system to be rotated whilst the amplifier and its associated equipment remain readily accessible. An aerial system embodying the invention has other advantages. For example, manufacturing costs are reduced, and manufacture facilitated, by use of a symmetrical paraboloidal main reflector surface. As compared with systems having an offset paraboloidal main reflector ,(in which the aerial periphery is elliptical, and the eifective aperture size determined by the minor axis of the ellipse), an aerial embodying the invention has a circular periphery and aperture the whole of its area can be efficiently utilised. Further, in aerial systems according to the invention, because the primary feed is directed towards the cold sky, any spillover past the sub-reflector results in a reduced noise temperature. The noise temperature of a system embodying the present invention represents a considerable reduction from the level associated with conventional aerials, together with a lower electrical loss in waveguide between the primary 1.? feed and the associated amplifier as well as the advantage that the amplifier itself remains accessible during tilting of the main reflector.

By way of example, an embodiment of the invention will be described in greater detail with reference to the accompanying drawings, in which:

FIG. 1 is a schematic front view of the aerial system,

FIG. 2 is a diagrammatic section of the line IIII in FIG. 1, and

FIG. 3 illustrates the geometry of the sub-reflector of the aerial system shown in FIGS. 1 and 2.

The aerial system shown in FIGS. 1 and 2 is a Cassegrain-derived system having a symmetrical paraboloidal \main reflector 1 and a sub-reflector 2 which is illuminated from a conical primary feed horn 3, the primary feed axis being offset from that of the main reflector 1. The subrefiector 2 is part of the surface of a hyperboloid having one focus f1 coincident with the focus F of the main reflector 1 and its other focus f2 depressed relative to the main reflector axis and located at the phase centre of the primary feed horn 3.

The sub-reflector periphery is bounded by the solid angle subtended between the focus F and the periphery of the main reflector 1. In the plane containing the primary feed axis and the principal axis of the paraboloid, the subreflector section has an offset, hyperbolic shape whilst in the orthogonal plane the section is a symmetrical hyperbola.

The sub-reflector 2 is supported by members 4 from the main reflector 1 which itself is supported (by means not shown) on a turntable platform 5. The conical primary feed is connected via a waveguide feeder 6 to a low-noise maser receiving amplifier 7.

The aerial system has an elevation axis 8 passing through aimplifier 7 perpendicularly to the primary feed axis, the reflectors being tiltable about the elevation axis by an elevation motor 9 and drive 10'. The amplifier 7 and drive motor 9 are housed in a cabin 11 on the platform 5.

The relation between the main and sub-reflectors is such that energy received by the aerial and reflected from the main reflector 1 towards the sub-reflector 2 is directed by the sub-reflector to, and focussed at the phase centre of, the primary feed device 3. The mean direction of such energy directed from the sub-reflector to the primary feed device thus is inclined to the axis of sy metry of the main reflector 1.

The geometry of the offset sub-reflector is illustrated in FIG. 3 which shows the generating hyperbola of the sub-reflector 2 and its relation to the phase centre of the primary feed. Curves W are spherical wavefronts of the energy reflected from the sub-reflector towards the main reflector (not shown).

In order to determine the sub-reflector profile it is convenient to consider that hyperbolic trace through the axis of the hyperboloid which is in the plane orthogonal to the plane containing the axis of the hyperboloid and the main reflector axis. In FIG. 3 this plane is normal to the paper and contains the line fl- Z.

The half-angle V) subtended at the focus F by the main reflector in that plane is given by where 2tp=aperture angle of the main reflector a=offset angle of sub-reflector axis.

The eccentricity (e) of the hyperboloid of revolution is obtained from the relationship:

tan \1 2 e1 The profile of the hyperbola can now be obtained and when referred to the apex of the curve is given by:

y 1) +fc where f is the distance between f and f Simple relationships can be used to determine the size of sub-reflector 2, horn 3 and the distance (f between f and f For example, the minimum distance between the primary horn and the sub-reflector to generate a spherical wavefront at the sub-reflector may be given by: where 2a:diameter of primary horn aperture t=wavelength The 10d half-beamwidth 5), in degrees, of a conical horn radiation pattern is given approximately by:

To avoid scattering from the sub-reflector due to diffraction losses, it is desirable that its maximum dimension d (FIG. 1), in a plane perpendicular to planes containing the main reflector axis, and to which the main At 4000 mc./s., the following dimensions are therefore obtained, Wavelength, \=7.5 cm. (2.95 in.) Distance between foci, f =5.89 m. (19 ft., 4 in.)

Diameter of horn aperture, 2a=66.2 cm. (26.1 in.) Minimum cross-section of sub-reflector, d=1.5 m. (59

These dimensions are by way of example and can be adjusted in accordance with individual requirements.

The aperture angle 21p of the main reflector can be designed to allow the aperture plane of the primary horn to coincide with the reflector profile.

For example, since the distance (r) from the focus F of the paraboloid to its surface, measured along the line f -f is given by D sec g 7 (a) 4 tan 2 D being the diameter of the main reflector,

D=70 ft., when r(a)=f =19 ft., 4 in. ot=20 than the aperture angle of the reflector becomes An improvement in aerial elficiency can be obtained by modifying the shape of the sub-reflector 2 to spread energy radiated from the feed 3 out towards the periphery of the main reflector 1, so as to approach the ideal of uniform illumination of the main reflector. In order to achieve a uniform phase front, the main reflector shape also would require modification.

The aerial system described with reference to FIGS. 1 and 2. possesses a number of advantages both from manufacturing and operational standpoints.

From the manufacturing point of view, the main reflector dish is symmetrical and thus easier and more economical to construct than a paraboloidal reflector which is offset from the main axis of the paraboloid and therefore asymmetrical. In the present system, it is the subreflector which is asymmetrical and, apart from other advantages to be discussed, this is relatively easier to construct since it is of smaller size.

The size of the sub-reflector is dependent on the tolerable wind loading at the prime focus F of the system and governs the location and aperture of the primary feed conical horn 3 necessary to achieve a spherical wavefront. The fundamental design consideration of the primary feed is to achieve maximum gain factor, since due to the siting of the sub-reflector 2 any spillover past the latter is at the temperature of the cold sky and does not, even at low elevations of the aerial suffer interference from other stations or degrade the performance of the low-noise receiver amplifier 7, the latter being located behind the primary feed phase centre f2.

Since the primary feed axis passes through the mechanical axis of elevational rotation 8, it is possible to connect the feed horn 3 directly to the receiver 7 by a straight length of overmoded waveguide 6, without using special transducers at the feed end. The lack of a transducer at the feed end would also reduce the problems of unwanted mode resonances.

It is desirable, for operational purposes, to accommodate a standby low-noise receiver amplifier and possibly amplifiers operating in different frequency bands. With the construction shown in FIGS. 1 and 2, this could be achieved by locating other low-noise amplifiers with associated feeder waveguides and feed horns with the relevant primary feed axes directed towards the sub-reflector with their phase centres on the circle of rotation of the phase centre 2 about the main reflector axis. In order to switch from one receiver to another, the sub-reflector 2 would be rotated to bring its focus f1 on the relevant primary feed axis associated with the selected amplifier. No additional waveguide switching would be necessary. In a particular arrangement, catering for a main and a standby low-noise amplifier, each is located on the elevation axis and each has its own feed horn and feeder waveguide.

Since the feed horn 3 will provide a fairly narrow cone of illumination, the horn aperture is of suflicient size to accept auxiliary tracking modes. Using an overmoded feeder waveguide, as discussed above, the necessary mode extraction can be accomplished at the receiver end. The aerial system described with reference to FIGS. 1 and 2 also permits the use of lobing tech niques.

The aerial system described with reference to FIGS. 1 and 2 enables a far shorter waveguide feeder to be used as compared with a centre fed aerial and the relatively high-level forward spillover normally associated with a Cassegrain configuration has been directed towards the cold temperature sky, without the use of a relatively expensive oflset paraboloidal main reflector. In addition, it permits a large aerial construction having an elevation axis located at the lower end of the main reflector, resulting in advantages associated with the maintenance of associated equipment, particularly of the low-noise receiver amplifier since this can now be located on the elevation axis.

In the above description, reference to electrical equipment associated with the aerial has been confined to that which is directly relevant to the invention, and the functional description has been in respect of use of the aerial as a receiving aerial. However, as is usual, equipment additional to that described will normally be associated with the aerial, including azimuth drive equipment and transmitting equipment operable to feed signals to the primary feed 3 for transmission by the aerial. This additional equipment is indicated by legends in FIG. 2.

What is claimed is:

1. An aerial system including a paraboloidal main reflector symmetrical about the axis of its generating parabola, and a sub-reflector supported by the main reflector and positioned to direct energy received by reflection from the main reflector to a primary feed device, the subreflector having a reflecting surface defined by part of a hyperboloidal surface one focus of which coincides with the focus of the main reflector and the other of which is nearer to the main reflector and offset from the axis of the main reflector such that in the plane containing the axis of the said hyperboloid and the axis of the said paraboloid the sub-reflector has an offset hyperbolic shaped section and in a plane orthogonal to the first said plane the sub-reflector has a section which is a symmetrical hyperbola, and in which the primary feed device has a phase centre at the said other focus of the said hyperboloid.

2. An aerial system including a paraboloidal main reflector symmetrical about the axis of its generating parabola, a sub-reflector having a reflecting surface defined by the rotation of part of a hyperbola defined by the equation y -1) +fc in which x and y are the coordinates of the hyperbola referred to the apex of the hyperbola,

e is the eccentricity of the hyperboloid of revolution, and

f is the distance between the foci of the generating hyperbola, the sub-reflector having a first focus located approximately at the focus of the main reflector and a second focus depressed relative to the said paraboloid axis and located nearer the main reflector surface, and

a primary feed device for illuminating the sub-reflector,

the said feed device having a phase centre at the said other focus.

3. An aerial system according to claim 2, in which the maximum dimension d of the sub-reflector in a plane perpendicular to a plane containing the main reflector axis and a plane to which the main reflector axis is perpendicular, is not greater than twenty times the operating wavelength of the aerial system.

4. An aerial system according to claim 1, in which the maximum dimension d of the sub-reflector in a plane perpendicular to a plane containing the main reflector axis and a plane to which the main reflector axis is perpendicular, is not greater than twenty times the operating wavelength of the aerial system.

5. An aerial system including a main reflector in the form of a paraboloid of revolution symmetrical about its axis mounted on a base member, a sub-reflector located adjacent to the focus of the main reflector, the subreflector being in the form of part of a hyperboloid of revolution produced by rotation about the axis joining the foci, one focus of which is coincident with that of the paraboloid, and a primary feed device for illuminating the sub-reflector having a phase centre on the other focus of the hyperboloid, characterised in that the axis of the hyperboloid is inclined to that of the paraboloid with the other focus of the hyperboloid below the axis of the paraboloid and close to the main reflector so that the primary feed device is placed some distance below the centre of the main reflector.

6. An aerial system according to claim 5, in which the main reflector has an aperture angle such that the aperture of the primary feed device coincides with the main reflector profile.

7. An aerial system according to claim 5, including means for rotating the main reflector and sub-reflector together in elevation about an axis passing through the main axis of the hyperboloid of which the sub-reflector forms part.

8. An aerial system according to claim 7, and including a straight length of waveguide connecting the primary feed device directly to a receiver, and in which the said elevation axis passes through the said receiver.

9. An aerial system according to claim 5, including at least one further primary feed device for illuminating the sub-reflector, the or each further primary feed device having a phase centre disposed on the circle of rotation about the axis of the main reflector, of the said other focus of the said hyperboloid, and means for rotating the sub-reflector so as to switch between the different primary feed devices.

References Cited UNITED STATES PATENTS 3,332,083 7/1967 Broussaud 343781 X 3,414,904 12/1968 Ajioka 343781 ELI LIBERMAN, Primary Examiner M. NUSSBAUM, Assistant Examiner US. 01. X11. 

